This invention relates to a method and apparatus for manufacturing an aluminum alloy wire that is particularly suitable for use in conducting electricity. The wire produced by the method and apparatus of this invention has improved properties of yield strength, ultimate tensile strength, percent ultimate elongation, ductility, fatigue resistance and creep resistance as compared with conventional aluminum alloy electrical conductors of similar electrical properties.
In recent years the use of aluminum as an electrical conductor has increased significantly. An electrical grade conductor with a minimum of 99.45% aluminum was first used for overhead transmission lines in the early 1890's and has been used extensively since then with great success. There are other electrical applications where aluminum could be used only if certain physical and mechanical properties are achieved. These include building wire, telephone cable, battery cable, automotive harness wiring, aircraft cable, transformer wire, magnet wire and appliance cord. Inspection of these uses indicates that a material which possesses high strength and a high degree of connectability, coupled with a minimum loss in electrical conductivity, would be required for successful performance.
Electrical Conductor grade aluminum, in the fully annealed condition, possesses acceptable ductility and electrical conductivity. However, it is seriously handicapped by its poor mechanical properties and thermal stability. This precludes its use in applications where a strong, reliable connection is required. The connection or termination of the system is one of the most critical parts of any electrical system. The termination or connection is also the part that is handled by the public, and consequently is very often subjected to careless or poor workmanship. An ideal system would consist of conductor and termination designed in such a way that it would produce a "fool proof" system.
One of the integral components of the system, the conductor itself, could be made stronger and with high thermal stability simply by alloying the aluminum with magnesium, silicon, copper, etc., as has been done in the past for many structural applications. However, the decrease in electrical conductivity associated with the high solubility of these alloying additions prohibits their use in electrical conductor aluminum in more than very small amounts. Another way that the mechanical properties of the aluminum can be increased is to subject it to a certain amount of cold work in order to produce extensive work hardening in the matrix. This method, however, will render the aluminum unusable as it yields an unstable cold worked structure with both low ductility and extremely low thermal stability.
A method for improving the physical properties of an aluminum alloy without seriously affecting the electrical properties thereof was disclosed in U.S. Pat. No. 3,920,411 of which copending application Ser. No. 632,982, abandoned was a continuation-in-part. The method disclosed therein consisted of alloying from about 0.35 to about 4.0 weight percent cobalt, from about 0.1 to about 2.5 weight percent iron, the remainder being aluminum with associated trace elements, and thereafter continuously casting, hot-working, cold-working without preliminary or intermediate anneals, and thereafter annealing the product to achieve an electrical conductivity about 61% IACS and improved mechanical properties as compared with conventional electrical conductors.
It is an object of this invention to yet further improve the mechanical properties of an aluminum alloy electrical conductor by more closely controlling the thermo-mechanical processing steps broadly disclosed in the aforementioned U.S. Pat. No. 3,920,411, thereby obtaining a fine, stable cell structure in the aluminum matrix containing a fine dispersion of stable, insoluble intermetallic phase particles.
It has been known for some time that aluminum and its alloys develop a well-defined cell structure when subjected to various degrees of deformation. This is attributed to the high stacking fault energy of aluminum which by the prevention of dislocations splitting into partials, aids in the cross-slip process necessary for subgrain formation. During deformation, the dislocation density increases and well-defined cells are formed until an equilibrium cell size and dislocation density is reached.
Moreover, the prior art has long recognized that the strength of metal is inversely proportional to the size of the grains therein. The effect of grain size on the yield strength of metal was first studied by Hall in 1951 and Petch in 1953 in iron. Their experimental results could be described by a relationship of the type EQU .sigma.=.sigma..sub.o +k d.sup.-1/2
where .sigma. is the yield strength, .sigma..sub.o the frictional stress, and d the grain size. Several investigations have been carried out on the effect of subgrain size on the yield strength of different materials and also found it to obey a Hall-Petch type relation.
Because of the tendency of subgrains to coalesce during recovery and recrystallization, thereby growing in size and thus promoting a decrease in the yield strength of the metal, the prior art recognized that it would be advantageous to provide intermetallic precipitates in the aluminum matrix which could pin dislocation sites between adjacent subgrain boundaries, thereby immobilizing the grain boundaries by hindering the rearrangement of dislocations and therefore inhibiting the movement of the recrystallization front. Accordingly, such precipitates, as discussed in the aforementioned U.S. Pat. No. 3,920,411, could effectively limit the subgrain growth and thus render the physical properties of the metal more stable at elevated temperatures.
As previously mentioned, the conductor of the aforementioned U.S. Pat. No. 3,920,411 is formulated from an aluminum based alloy prepared by mixing cobalt, iron and optionally other alloying elements with aluminum in a furnace to obtain a melt having requisite percentages of elements. The aluminum content of the alloy could vary from about 93.50 percent to about 99.65 percent by weight. The optional alloying element or group of alloying elements could be present in a total concentration of up to 2.50 percent by weight, preferably from 0.1 percent to about 1.75 percent by weight.
After preparing the melt, the aluminum alloy was continuously cast into a continuous bar by a continuous casting machine and then, substantially immediately thereafter, hot-worked in a rolling mill to yield a continuous aluminum alloy rod.
As further described in the aforementioned patent, a continuous casting machine serves as a means for solidifying the molten aluminum alloy metal to provide a cast bar that is conveyed in substantially the condition in which it solidified from the continuous casting machine to the rolling mill, which serves as a means for hot-forming the cast bar into rod or another hot-formed product in a manner which imparts substantial movement to the cast bar along a plurality of angularly disposed axes.
The continuous casting machine is of conventional casting wheel type having a casting wheel with a casting groove in its periphery which is partially closed by an endless belt supported by the casting wheel and an idler pulley. The casting wheel and the endless belt cooperate to provide a mold into one end of which molten metal is poured to solidify and from the other end of which the cast bar is emitted in substantially that condition in which it is solidified.
The rolling mill is of conventional type having a plurality of roll stands arranged to hot-form the cast bar by a series of deformations. The continuous casting machine and the rolling mill are positioned relative to each other so that the cast bar enters the rolling mill substantially immediately after solidification and in substantially that condition in which it solidified. In this condition, the cast bar is at a hot-forming temperature within the range of tempertures for hot-forming the cast bar at the initiation of hot-forming without heating between the casting machine and the rolling mill. In the event that it is desired to closely control the hot-forming temperature of the cast bar within the conventional range of hot-forming temperatures, means for adjusting the temperature of the cast bar may be placed between the continuous casting machine and the rolling mill without departing from the inventive concept disclosed herein.
The roll stands each include a plurality of rolls which engage the cast bar. The rolls of each roll stand may be two or more in number and arranged diametrically opposite from one another or arranged at equally spaced positions about the axis of movement of the cast bar through the rolling mill. The rolls of each roll stand of the rolling mill are rotated at a predetermined speed by a power means such as one or more electric motors and the casting wheel is rotated at a speed generally determined by its operating characteristics. The rolling mill serves to hot-form the cast bar into a rod of a cross-sectional area substantially less than that of the cast bar as it enters the rolling mill.
The peripheral surfaces of the rolls of adjacent roll stands in the rolling mill change in configuration; that is, the cast bar is engaged by the rolls of successive roll stands with surfaces of varying configuration, and from different directions. This varying surface engagement of the cast bar in the roll stands function to knead or shape the metal in the cast bar in such a manner that it is worked at each roll stand and also to simultaneously reduce and change the cross-sectional area of the cast bar into that of the rod.
As each roll stand engages the cast bar, it is desirable that the cast bar be received with sufficient volume per unit of time at the roll stand for the cast bar to generally fill the space defined by the rolls of the roll stand so that the rolls will be effective to work the metal in the cast bar. However, it is also desirable that the space defined by the rolls of each roll stand not be overfilled so that the cast bar will not be forced into the gaps between the rolls. Thus, it is desirable that the rod be fed toward each roll stand at a volume per unit of time which is sufficient to fill, but not overfill, the space defined by the rolls of the roll stand.
As the cast bar is received from the continuous casting machine, it usually has one large flat surface corresponding to the surface of the endless band and inwardly tapered side surfaces corresponding to the shape of the groove in the casting wheel. As the cast bar is compressed by the rolls of the roll stands, the cast bar is deformed so that it generally takes the cross-sectional shape defined by the adjacent peripheries of the rolls of each roll stand.
Thus, it will be understood that with this apparatus, cast aluminum alloy rod of an infinite number of different lengths is prepared by simultaneous casting of the molten aluminum alloy and hot-forming or rolling the cast-aluminum bar.
According to the method described in the aforementioned patent, the continuous rod was cold-drawn through a series of progressively constricted dies, without intermediate anneals, to form a continuous wire of desired diameter. Thereafter, the wire was annealed or partially annealed to obtain a desired tensile strength and cooled. The annealing operation was disclosed as being continuous as in resistance annealing, induction annealing, convection annealing by continuous furnaces or radiation annealing by continuous furnaces, or, preferably, batch annealed in a batch furnace.
In order to produce a product having improved percent ultimate elongation, increased ductuity and fatigue resistance, and increased electrical conductivity in accordance with the objects of the aforementioned patent, it was necessary to anneal at temperatures of about 450.degree. F. to about 1200.degree. F. when continuously annealing with annealing times of about 5 minutes to about 1/10,000 of a minute. On the other hand, when batch annealing, a temperature of approximately 400.degree. F. to about 750.degree. F. was employed with resident times of about 30 minutes to about 24 hours.
Prior art systems for the continuous production of rod from molten metal, i.e., systems where the cast bar is delivered substantially immediately to the rolling mill without an intervening homogenizing step such as described above, typically provide a reduction of less than 30% in the first stand of the rolling mill. Reduction of 20% and 25% are conventional. Upon observation, applicants have found that such a cast bar does not exhibit a clearly defined subgrain structure after that degree of deformation, but rather that the matrix is substantially free of subgrains and that at most there is a randomly disposed arrangement of very large ragged cells.
While a well defined subgrain structure will, of course, be formed during subsequent deformations in prior art systems, the stock product rolled under such conditions is at a disadvantage because the subgrain structure, which becomes broken-up and refined when undergoing subsequent deformations, is deprived of the refining effects of the initial roll stands under which it exhibited an insufficiently-formed subgrain structure. Moreover, a stock product which does not exhibit a well defined subgrain structure after the first deformation undergoes a lesser degree of dynamic recrystallization in the hot-forming process than a stock product in which the subgrain structure is formed after the first deformation. This phenomenon is attributable to the fact that the product is moving at higher speeds and undergoing increased cooling in the latter stages of the rolling mill than in the early stages thereof. Consequently, if the subgrain structure is not sufficiently formed until after the speed and the cooling rate reach critical points, dynamic recrystallization will not take place. Accordingly, the ductility of the stock will be diminished and the finished product will have a lower elongation than a product which undergoes a greater degree of dynamic recrystallization during hot-forming.
It is, therefore, an object of this invention to manufacture an aluminum alloy electrical conductor in a system which includes continuous casting and hot-forming in a series of deformities, and wherein a sufficient degree of deformation is provided in the first of the series of deformations so as to therein form a substantially well-defined subgrain structure in the stock product which will be broken-up and thus refined in subsequent deformations, and which will permit dynamic recrystallization of the product during hot-forming, thereby improving the ductility of the stock.
In accordance with this invention, it has been determined that a reduction of more than 30% in the first roll stand is necessary to achieve the subgrain structure necessary to accomplish the foregoing. In a preferred embodiment of the invention the reduction is at least 37%.